In fabricating a semiconductor device, a method of introducing a substrate with a gas into a reaction chamber under high vacuum and irradiating the gas as a molecular beam in the reaction chamber is widely applied as a method of producing a high reaction efficiency. When controlling the reaction, since the gas is supplied in a molecular beam, it is necessary to control the pressure of the supply gas accurately at a very low pressure less than 10.sup.-5 Torr. In the prior art, a mass flow controller, a leak valve, or the like are used for this control. Pressure control systems utilizing a mass flow controller and a leak valve, respectively, are illustrated in FIGS. 11 and 12, respectively.
In FIG. 11, reference numeral 1 designates a reaction chamber, reference numeral 2 designates a mass flow controller, reference numeral 3 designates material gas, and reference numeral 4 designates a substrate. In the first prior art example, the pressure of the material source gas introduced into the reaction chamber 1 is controlled by the mass flow controller 2. However, since the limit of flow rate controllability of a mass flow controller is generally around 1 SCCM, the pressure of the material source gas introduced into the reaction chamber 1 can be reduced only to a pressure of about 10.sup.-5 Torr in the mass flow controller 2. Therefore, the mass flow controller is not applicable to an MBE apparatus which is required to be used at a pressure of 10.sup.-8 .about.10.sup.-10 Torr.
FIG. 12 shows a system utilizing a control leak valve. In FIG. 12, reference numeral 5 designates a pressure gauge, reference numeral 6 designates a control leak valve for low pressure control which adjusts the aperture of the valve based on the pressure that is measured with the pressure gauge 5. In the second prior art example, since pressure control is performed by aperture adjustment with a vernier of the control leak valve 6, its controllability depends on the machined precision of an orifice and a vernier. Therefore, when very low pressure control is to be performed, an extreme machine precision is required, and reproducibility and durability are not satisfactory.
In a gas branch piping which changes a gas passage, a valve is conventionally used as a gas branching means. However, since changing of a gas passage by the valve is carried out by mechanical control, when the gas passage is changed quite often, controllability and durability are not satisfactory.
Since the prior art MBE apparatus utilizes the apparatus described above as flow rate control apparatus, precision, reproducibility, and durability of the flow rate control apparatus are not satisfactory.
Furthermore, since the prior art gas branch piping apparatus utilizes valves as a gas branching means, when the gas passage is changed quite often, controllability and durability of the gas branching means are not satisfactory.